During operation of the internal combustion engine, the catalyzer is heated by means of a heat exchange with the exhaust gases of the engine and is maintained at its operating temperature. After the cold engine is started, a certain time passes until the catalyzer reaches this temperature. In this time, the conversion rate of the catalyzer is not optimal and a high emission of unwanted exhaust-gas components is caused thereby. A reduction of the unwanted emission can be obtained by shortening the time duration which is necessary for reaching the operating temperature of the catalyzer.
The article of Zahn et al entitled "Die Abgasreinigung der neuen Mercedes-Benz 300 SL-24 und 500 SL - Aufbau und Wirkungsweise" printed in the technical journal "MTZ", volume 6, (1989) discloses a method for shortening this time span. In this method, the exhaust-gas temperature is increased after a cold start by adding air behind the exhaust valves and by intervening in the ignition timing. In this way, a larger amount of heat is conducted to the catalyzer under conditions which are otherwise the same thereby accelerating the temperature increase of the catalyzer.
Also, for motor vehicles equipped with an automatic transmission and having the same performance, it is known to increase the level of the rotational speed of the engine after a cold start by changing the shifting program. The increase of the heat quantity transported with the exhaust-gas flow likewise effects an acceleration of the temperature increase of the catalyzer. Such a method is disclosed, for example, in the article of StrauB et al entitled "Einspritz- und Zundsysteme der neuen Mercedes-Benz Vierventilmotoren for den SL-Roadster" published in the journal "MTZ", volume 7/8 (1989).
One disadvantage of the known methods is that the measures for increasing exhaust-gas temperature are applied only during cold start and in the warm-up phase. For this reason, in the case of a restart wherein the engine is still operationally warm such as during city traffic and repeated stops for such things as shopping wherein the engine is shut off for only a short time, still higher toxic emissions can occur. This is caused by the different rapid rates of cooling of the engine on the one hand and the exhaust-gas system containing the catalyzer on the other hand.
The temperature of the catalyzer drops relatively rapidly compared to the compact engine block. For this reason, a deterioration of the conversion rate when restarting occurs without initiating measures of the above-mentioned methods for accelerating a reheating of the catalyzer.